Physiology - respiratory Flashcards
What is internal respiration?
The intracellular mechanisms which consume oxygen and produce carbon dioxide
What is external respiration?
The sequence of events that lead to the exchange of oxygen and carbon dioxide between the external environment and the cells of the body
What are the 4 steps of external respiration?
1 - ventilation (gas exchange between atmosphere and alveoli)
2 - gas exchange between alveoli blood
3 - gas transport in the blood
4 - gase exchange between the blood and tissues
What is Boyle’s law?
At any constant temperature the pressure exerted by a gas varies inversely with the volume of the gas - as the volume of a gas increases the pressure exerted by the gas decreases
The intra-alveolar pressure must become less than atmospheric pressure for air to flow into the lungs during inspiration. How is this achieved?
Before inspiration the intra-alveolar pressure is equal to atmospheric pressure. During inspiration the thorax and lungs expand as a result of contraction of inspiratory muscles causing the intra-alveolar pressure to fall (the air molecules become contained in a larger volume)
What are the 2 forces that hold the thoracic wall and the lungs in close opposition?
1) the intrapleural fluid cohesiveness
2) the negative intrapleural pressure
What muscle causes the volume of the thorax to increase vertically?
Contraction of the diaphragm
What muscle controls the bucket handle mechanism?
External intercostal muscles lifts the ribs and moves the sternum out
How does expiration come about?
Passive process brought about by relaxation of inspiratory muscles - the chest wall and lungs recoil to their preinspiration size
This recoil causes the intra-alveolar pressure to rise which forces air to leave the lungs until the intra-alveolar pressure becomes equal to the atmospheric pressure
Why does lung collapse occur in pneumothorax?
Air escapes into the pleural space which can abolish the transmural gradient causing the lung to collapse
What causes the lungs to recoil during expiration?
Elastic connective tissue in the lungs and the alveolar surface tension
Smaller alveoli have a higher tendency to collapse, according to what law?
LaPlace law
What prevents smaller alveoli from collapsing?
Pulmonary surfactant - secreted by type II alveoli
It lowers alveolar surface tension (lowers the surface tension of smaller alveoli more than larger alveoli)
This prevents the smaller alveoli from collapsing and emptying their air contents into the larger alveoli
Why does respiratory distress syndrome of the new born happen in premature babies?
Developing fetal lungs cannot make surfactant until late in pregnancy so premature babies may not have enough surfactant to combat the alveolar surface tension
Baby will make very strenuous inspiratory efforts to try and overcome the high surface tension and inflate the lungs
What is another factor that helps keep the alveoli open?
Alveolar interdependence - if an alveolus starts to collapse, the surrounding alveoli are stretched and then recoil which exerts expanding forces in the collapsing alveoli to open it
In summary, what forces keep the alveoli open and what promote closure of alveoli?
Open - transmural pressure gradient, pulmonary surfactant and alveolar interdependence
Closed - elasticity of stretched lung connective tissue, alveolar surface tension
What are the major inspiratory muscles?
Diaphragm and external intercostal muscles
What are the accessory muscles of inspiration?
Sternocleidomastoid, scalenus and pectoral muscles
What are the muscles of active expiration?
Abdominal muscles and internal intercostal muscles
Volume of air entering or leaving the lungs during a single breath. What lung volume is this and what is it’s average value?
Tidal volume, typically 0.5L
What is the inspiratory reserve volume (IRV)?
Extra volume of air that can be maximally inspired over and above the typical resting tidal volume, typically 3L
What is the expiratory reserve volume (ERV)?
Extra volume of air that can be maximally expired beyond the normal volume of air after a resting tidal volume, typically 1L
What is the residual volume (RV)?
Minimum volume of air remaining in the lungs even after a maximal expiration, typically 1.2L
What is the inspiratory capacity?
Maximum volume of air that can be inspired at the end of a normal quiet expiration (IC = IRV +TV), typically 3.5L
Volume of air in the lungs at the end of normal passive expiration. What is this lung volume and what is it’s average value?
Functional residual capacity (FRC = ERV + RV), typically 2.2L
What is the vital capacity?
Maximum volume of air that can be moved out during a single breath following a maximal inspiration (VC = IC (IRV + TV) + ERV), typically 4.5L
What is the total lung capacity?
Total volume of air the lungs can hold
TLC = VC + RV, typically 5.7L
When would the residual volume increase?
When the elastic coil of the lungs is lost e.g. in emphysema
What is the FVC?
Forced vital capacity - maximum volume that can be forcibly expelled from the lungs following a maximal inspiration
What is the FEV1?
Forced expiratory volume in one second - volume of air that can be expired during the first second of expiration in an FVC determination
What is a healthy FEV1/FVC ratio normally?
> 70%
What happens to the FEV1/FVC ratio in obstructive lung disease and why is this?
<70% because the FVC is normal or low and the FEV1 is low
What happens to the FEV1/FVC ratio in restrictive lung disease and why is this?
Normal >70%, both the FVC and FEV1 will be reduced, making the ratio normal
What does parasympathetic innervation do to the bronchi?
Causes bronchoconstriction
What does sympathetic innervation do to the bronchi?
Causes bronchodilatation
In obstructive lung disease is inspiration or expiration more difficult and why?
Expiration is more difficult than inspiration because during expiration the pleural pressure rises causing compression of the alveoli and airways
Diseased airways are also more likely to collapse
What happens if a patient had restrictive airways disease on top of obstructive?
The problem of expiration becomes worse because the patient will have decreased elastic recoil of the lungs e.g. COPD - obstructed airway and emphysema (restrictive)
What is pulmonary compliance?
Measure of the effort that has to go into stretching or distending the lungs
The less compliant the lungs are, the more work is required to produce a given degree of inflation
What can cause decreased pulmonary compliance?
Pulmonary fibrosis, pulmonary oedema, lung collapse, pneumonia, absence of surfactant
What does decreased pulmonary compliance cause clinically and what pattern would it show on spirometry?
Decreased pulmonary compliance means greater change in pressure is needed to produce a given change in volume i.e. lungs are stiffer. This causes shortness of breath, especially on exertion
Decreased pulmonary compliance may cause a restrictive pattern on spirometry
What could cause increased pulmonary compliance?
If the elastic recoil is lost, occurs in emphysema. Patients have to work harder to get the air out of the lungs - hyperinflation of lungs
Compliance also increases with increasing age
When does the work of breathing increase?
Pulmonary compliance decreased, airway resistance increased, elastic recoil, decreased, need for increased ventilation
What is the anatomical dead space?
Inspired air that remains in the airways where it is not available for gas exchange
What is the pulmonary ventilation?
Volume of air breathed in and out per minute
Tidal volume x respiratory rate = 6L/min
What is the alveolar ventilation?
Volume of air exchanged between the atmosphere and alveoli per minute
(Tidal volume - anatomical dead space) x respiratory rate = 4.2L/min
How is pulmonary ventilation increased?
By increased both the depth and rate of breathing but due to the dead space it is more beneficial to increase the depth of breathing
What is ventilation and perfusion?
Ventilation is the rate at which gas is passing through the lungs
Perfusion is the rate at which blood is passing through the lungs
Blood flow (perfusion) and ventilation vary from the bottom to the top of the lungs. What is the result of this?
The average arterial and alveolar partial pressures of oxygen are not exactly the same.
This is usually not significant but can be in disease
What is the alveolar (not anatomical) dead space?
Ventilated alveoli which are not adequately perfused with blood are considered as alveolar dead space
In healthy people, this is very small and of little importance but in disease the alveolar dead space could increase significantly
What happens as a result of increased perfusion?
Accumulation of carbon dioxide in alveoli due to increased perfusion decreases airway resistance leading to increased airflow
What happens as a result of increased ventilation?
Increase in alveolar oxygen concentration due to increased ventilation causes pulmonary vasodilation which increases blood flow to match larger airflow
When is perfusion greater than ventilation?
When carbon dioxide levels in the alveoli increase or when the oxygen levels in the alveoli decrease
When is ventilation greater than perfusion?
When carbon dioxide levels are low or oxygen levels are high in the alveoli
What happens to the pulmonary and systemic circulation when there is decreased oxygen?
Pulmonary arterioles constrict and systemic arterioles dilate
What happens to the pulmonary and systemic circulation when there is increased oxygen?
Pulmonary arterioles dilate and systemic arterioles constrict
What 4 factors influence the rate of gas exchange across the alveolar membrane?
- partial pressure of oxygen and carbon dioxide
- diffusion coefficient for oxygen and carbon dioxide
- surface area of alveolar membrane
- thickness of alveolar membrane
What is Dalton’s law of partial pressures?
The total pressure exerted by a gaseous mixture = The sum of the partial pressures of each individual component in the gas mixture
What is the partial pressure of a gas?
The pressure that one gas in a mixture of gases would exert if it were the only gas present in the whole volume occupied by the mixture at a given temperature
How is partial pressure of oxygen in the alveoli calculated?
PAO2 = PiO2 - (PaCO2/0.8)
PAO2 - partial pressure of O2 in alveoli
PiO2 - partial pressure of O2 in inspired air
PaCO2 - partial pressure of CO2 in arterial blood
What is the difference in diffusion coefficient between oxygen and carbon dioxide?
Diffusion coefficient for CO2 is 20 times that of O2
What is the typical gradient between alveolar and arterial PO2?
A small gradient between PAO2 and PaO2 is normal but a big gradient indicates problems with gas exchange in the lungs or right to left shunt in the heart
What is Fick’s law of diffusion?
The amount of gas that moves across a sheet of tissue in unit time is proportional to the area of the sheet but inversely proportional to its thickness
What type of cells are within the alveolar walls?
A single layer of type I alveolar cells
The amount of a given gas dissolved in a given type and volume of liquid (blood) at a constant temperature is proportional to the partial pressure of the gas in equilibrium with the liquid. What law is this?
Henry’s law
How is most O2 transported in the blood and what is the normal O2 concentration in arterial blood?
Mostly transported in the blood bound to Hb
Normal O2 concentration in the arterial blood is about 200ml/L
Oxygen in present in the blood in 2 forms, what are these forms?
Bound to Hb and physically dissolved
When might oxygen delivery to the tissues be impaired?
Respiratory disease, heart failure or anaemia
How does respiratory disease impair oxygen delivery to tissues?
Decreases arterial PO2 and hence decreases Hb saturation with O2 and O2 content of the blood
How does anaemia impair oxygen delivery to tissues?
Decreases Hb concentration and hence decreases O2 content of the blood
How does heart failure impair oxygen delivery to tissues?
Decreases cardiac output
What is the Bohr effect?
A shift of the oxygen haemoglobin dissociation curve to the right
Increased release of O2 by conditions at the tissues e.g. increase in hydrogen ions (decrease in pH), increase in carbon dioxide, increase in temperature and an increase in 2,3-BPG
